1. Field of the Invention
The present invention relates to a method of electrophotographically forming an image by using a dry developer material and, more particularly, to a method of stably forming an excellent electrophotographic image without performing cleaning.
2. Description of the Related Art
Image formation based on electrophotographic processes includes uniform charging of the surface of a photosensitive drum, formation of a latent image upon exposure of the charged surface, visualization (development) of the latent image with a toner, and transfer of the visualized image onto a paper sheet. The sheet onto which the visualized image has been transferred is separated from the photosensitive drum, and the transferred image is fixed. Thereafter, the copied sheet is delivered outside the system. The photosensitive drum after image transfer is prepared for the next image formation cycle.
In an apparatus for performing the above electro-photographic processes, since transfer efficiency of the visualized image onto the sheet is not 100%, some of toner particles remain on the surface of the photosensitive drum after image transfer is performed. The residual toner particles adversely affect image and formation must be removed prior to the next image formation cycle. In order to remove these residual toner particles, an exclusive cleaner has been used to scrape the residual toner particles from the photosensitive drum after the visualized image is transferred to the sheet. However, use of the exclusive cleaner is not preferable because it renders an image formation apparatus bulky.
In order to solve the above problem, there is proposed an image forming apparatus comprising a developing unit having a cleaner function. In an apparatus of this type, charging, exposure, development by a cleaner/developing unit, and transfer are performed upon one revolution of the photosensitive drum. The photosensitive drum is further rotated by one revolution to cause the cleaner/developing unit to remove the residual toner. In this apparatus, however, in order to prevent an afterimage from being formed on the non-cleaned surface of the photosensitive drum once the image is formed, the circumferential length of the photosensitive drum must be longer than the length of a sheet having a maximum size. For example, when the maximum sheet size is a B4 size, the length of this sheet is 364 mm. In this case, the circumferential length of the photosensitive drum is a sum of 364 mm and an extra length (normally, about 25 mm). This circumferential length corresponds to a diameter of about 120 mm, thus requiring a large photosensitive drum. In addition, when images are continuously formed by using this apparatus, one additional revolution of the drum to remove the residual toner prolongs a waiting period of the next copying sheet, thus lowering an image formation speed.
In order to eliminate the drawback of the above image forming system, there are proposed systems for performing development while residual toner particles are removed, as described in Japanese Patent Disclosure (Kokai) Nos. 54-109842 and 62-226173 and U.S. Pat. No. 4,265,998. Such a system does not require an exclusive cleaner, and cleaning need not be an independent step. Therefore, continuous image formation can be performed using a normal small-diameter photosensitive drum without causing the next copying sheet to wait for a period of time. However, an image forming apparatus employing the above system inevitably has a complicated developing unit and its associated structure as compared with the conventional electrophotographic image forming apparatus.
As described above, various improvements have been made for the conventional methods and apparatus for forming electrophotographic images. However, these improvements are based on the premise that toner particles remain on the surface of the photosensitive drum after the visualized image is transferred to the sheet.